Silicon-based photonic components working at 1.3 and 1.55-μm fiber-optic communications-wavelengths for fiber-to-home interconnects and local area networks (LAN) are a subject of intensive research because of the possibility of integrating optical elements and advanced electronics together on a silicon substrate using bipolar or complementary metal-oxide semiconductor (CMOS) technology. The resulting optoelectronic integrated circuit (OEIC) should exhibit a better performance than optical and electrical circuits when considered separately, and present a significantly lower cost than those based on III-V semiconductor materials.
Si passive structures, such as waveguides, couplers and filters have been extensively studied. Less work has been reported on Si active (or tunable) integrated devices such as modulators and switches, despite their importance as means of manipulating light beams for information processing (e.g., coding-decoding, routing, multiplexing, timing, logic operations, etc) in integrated-optic circuits. Some Si-based thermo-optic and electro-optic active devices have been demonstrated. In thermo-optic devices, the refractive index of Si is modulated by varying the temperature, inducing a phase modulation which in turn is used to produce an intensity modulation at the output of the device. For Si, the thermal change of the real optical refractive index is large. Nevertheless, the thermo-optic effect is rather slow and can only be used up to 1 MHz modulation frequencies. For higher modulation frequencies, up to few hundreds of MHz, electro-optic devices are required.
Most of the proposed electro-optic devices exploit the free carrier dispersion effect to change both the real refractive index and optical absorption coefficient. This is because the unstrained pure crystalline Si does not exhibit linear electro-optic (Pockels) effect and the refractive index changes due to the Franz-Keldysh effect and Kerr effect are very weak. In free-carrier absorption modulators (FCAM), changes in the optical absorption of the structure are directly transformed into an output intensity modulation. Phase modulation in a specific region of optical devices, such as Mach-Zehnder modulators, total-internal-reflection (TIR) based structures, cross-switches, Y-switches and Fabry-Perot (F-P) resonators, is also used to modulate the output intensity.
Free-carrier concentration in electro-optic devices can be varied by injection, accumulation, depletion or inversion of carriers. Si-based electro-optic modulators based on p-i-n diodes, metal-oxide-semiconductor field-effect-transistors (MOSFET) and bipolar-mode-field-effect-transistor (BMFET) structures have been proposed. Most silicon electro-optic intensity modulators and switches present some common features: they require long interaction distances and injection current densities higher than 1 kA/cm2 in order to obtain a significant modulation depth. Long interaction lengths are undesirable in order to achieve high levels of integration and miniaturization for fabricating low-cost compact chips. High current densities may induce thermo-optic effect due to heating of the structure, and cause an opposite effect on the refractive index change as that produced by free-carrier dispersion, reducing its effectiveness. There is therefore an urgent need, from the integration point of view, for structures that can be implemented in a micron-size region offering low current density, low power consumption and high-modulation-depth.